Conventionally, dye inks that use a dye for the color material have been used for the aqueous inks for ink jet recording. There are numerous advantages associated with the dye inks in that they exhibit excellent storage stability, hardly clog the nozzle of an ink jet printer, and the printed matter obtained therefrom is excellent in terms of gloss. However, their disadvantages originating from the dye, such as the poor weather resistance, which makes it impossible to store the printed matter for a long time, and inferior water resistance and light resistance have been pointed out. Accordingly, in recent years, aqueous inks for inkjet recording using a pigment that exhibits excellent weather resistance, water resistance and light resistance (hereafter, abbreviated as an aqueous pigment ink) have been developed.
An aqueous pigment ink is usually a pigment dispersion prepared by homogeneously dispersing pigment particles in an aqueous solvent such as water, and following the recording, forms an ink film containing the pigment particles on a recording medium. Image quality of the material recorded by the aqueous pigment ink largely depends on the color development and gloss of the pigment particles themselves in this ink film, and thus there has been a problem in that the color developing properties and gloss properties of the recorded portion are impaired due to the diffuse reflection of light by the pigment particles in the ink film, when a recording medium, such as a glossy paper, in which a recording surface is smoothed is used. Moreover, there have also been problems of insufficient fixation with respect to the recording medium and poor scratch resistance of the formed images.
In addition, ink jet printing is different from the usual printing method, and the printing process is conducted by discharging an ink from the printer head. The discharge system includes a piezo system and a thermal system. In the thermal system, in particular, since the ink inside the nozzle is boiled with a heater constituted of the heat elements provided in the printer head to discharge the ink droplets, a problem arises in that a phenomenon known as cogation occurs in the heater portion due to the rapid temperature change inside the nozzle so that the liquid is no longer discharged (discharge failure) during the prolonged printing process. Therefore, the ejection stability is also required for the ink to be used.
Currently, a water-based pigment ink that contains a resin component in order to improve the fixation onto the recording medium and the gloss has been studied, and an aqueous ink for inkjet recording to which aqueous polyurethane has been added is known as an ink to satisfy the physical properties to some extent. More specifically, an inkjet recording liquid containing a polyurethane that is formed by having an acetylene glycol type hydrophobic diol known as a surface active agent and a carboxyl group-containing diol as raw materials (for example, refer to Patent Document 1), an ink composition for inkjet printing containing an urethane-modified polyester resin (for example, refer to Patent Document 2), and an ink composition for inkjet printing containing a self-dispersing pigment and a polyurethane dispersing element (for instance, refer to Patent Document 3) have been known.
However, the inkjet recording liquid described in Patent Document 1 had a problem of inferior gloss properties, scratch resistance, or the like of the printed material, especially when a glossy paper was used as a recording medium. In addition, the ink composition for inkjet printing described in Patent Document 2 had problems of inferior ejection stability and poor abrasion properties of the obtained printed material, and the ink composition for inkjet printing described in Patent Document 3 had a problem of inferior ejection stability.    Patent Document 1: Japanese Unexamined Patent Application, First Publication No. Hei 06-279718    Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2004-300393    Patent Document 3: Published Japanese Translation No. 2005-515289 of the PCT International Publication